Eukaryotic chromosomes are organized and regulated on a variety of levels. A tiny, well-characterized Drosophila minichromosome called Dp1187 will continue to be used to investigate how chromosome structure is modulated during the process of Drosophila oogenesis. Dp1187 represents an extensively deleted X chromosome, the chromosome that normally organizes a specially active nucleolus in the nurse cells to nourish each growing egg. Although the rDNA genes themselves were deleted during the production of Dp1187, some of the X-specific 1.688 "complex" satellite that flanks the nucleolus organizer was retained. When analyzed by pulsed-field gel electrophoresis, the apparent size of the chromosome changes from 1300 kb to only about 680 kb or 400 kb during the development of the ovarian nurse (and follicle) cells. These changes are postulated to represent some vestiges of a normal process accompanying formation of the special nucleolus. They may also be related to the losses of heterochromatic DNA that accompany the growth of many polyploid and polytene cells, but whose cause is unknown. Physically mapping the structure of the chromosome during and after these changes, and observing the changes that occur in a collection of Dp1187 derivatives that were generated using transposable P elements, will determine if the alterations are caused by tissue-specific, breakable regions of special structure, or by the breakage and ligation of specific DNA sequences (DNA elimination). The function of these events during normal oogenesis will be addressed by characterizing genes required for the Dp1187 modifications. Several candidate genes that are known to be necessary for nurse cell nucleolus formation, or for normal nurse cell chromosome structure, will be tested. One gene, fs(2)4506, was already shown in preliminary experiments to block the production of the 680 kb and 400 kb band, and to disrupt nurse cell function. Finally, the relationship of the changes in Dp1187 chromosome structure to the phenomenon of position-effect variegation (PEV) will be analyzed. Genes on Dp1187 exhibit variegation, and flies bearing terminally Dp1187 derivatives show greatly enhanced variegation and their ovaries contain novel, 30-400 kb, heterogeneous molecules derived from the minichromosome. By analyzing the properties of Dp1187 derivatives bearing enhancer trap P elements at many different sites, and by cloning and characterizing the heterogeneous molecules, the importance of changes at the level of primary DNA sequence to PEV will be determined. Therefore, these studies will elucidate the structure and function of eukaryotic chromosomes, a subject of widespread importance to the genetics of all organisms, including humans.